Add system that can run scripts upon user creation

[selten]

Add config options to set an API key for a user automatically, add config options to setup Python scripts to execute upon user creation.
Update the UserManager to trigger a "post user creation" system, which is non-blocking to the user-creation process.
Add documentation for the new functionality.

This (mostly) resolves https://github.com/galaxyproject/galaxy/issues/10163.

[selten]

This should be adapted to be Celery compliant.